A horse is completely different from a human. A horse evolved from running away from giant saber toothed cats and other things. To do this they have a reserve of blood cells in their spleen that probably isn't healthy to use but is better than being dead.
Humans didn't evolve to escape, but to pursue. A human runs after a horse, choosing hot weather or deep snow, patiently jogging along behind it until the horse is exhausted and the human kills and eats it. We don't have extra blood to vent from our spleens, but we do have hairless skin that can outcool itself compared to a horse, and snowshoes too.
Running is the wrong sport for us. We should compete at persistence-hunting a simulated prey.